1. Field of the Invention
This invention relates to a method for the selective removal of the metal film layer accumulated during xe2x80x9clift offxe2x80x9d processing of substrates including wafers, integrated circuits, masks and the like using a stream of carbon dioxide spray.
2. Description of the Prior Art
In the processing of integrated circuits and the like, a common method for forming a patterned conductor layer on a substrate is a process which typically involves depositing a photoresist layer on the substrate which is patterned such that the photoresist layer covers the portions of the substrate which will not have the coating of conductor material. The photoresist layer, is generally exposed and developed such that when a metal layer is subsequently exposed over the entire surface, the portion of the metal layer deposited on the top of the photoresist layer is discontinuous from that deposited directly on the substrate (the conductor portions). This discontinuity is typically achieved by developing the pattern side walls of the photoresist defining uncovered areas with an undercut profile. To complete the patterning process, it is then necessary to remove the unwanted metal layer deposited upon the photoresist layer and subsequently the photoresist layer itself, leaving the conductor portions intact on the substrate.
The prior art reveals numerous methods for removal of the metal and photoresist layers during such processing. Prior known conventional methods involved soaking the substrates for many hours in a suitable solution before the desired metal layer could be removed so that the solution could penetrate the photoresist through the side walls since access to the photoresist is significantly blocked by the metal layer. Various methods are taught which attempt to increase the speed by which these layers may be removed. For example, U.S. Pat. No. 4,662,989 to Casey et al. teaches the application of an additional layer of material which has sufficiently different thermal expansion properties on the metal film layer so as to induce stress and cause cracks in the metal film layer for improved solvent penetration and thus more efficient and quicker dissolving of the photoresist layer.
U.S. Pat. No. 4,871,651 to McCune Jr. et al. involves subjecting an entire substrate to sufficiently low temperatures preferably by means of immersion in a bath of liquid nitrogen and thus extreme thermal stress so as to cause removal of the unwanted metal and photoresist layers due to the different thermal expansive properties of the photoresist layer versus the substrate, causing tensile cracking at the photoresist/substrate interface. Although the McCune Jr. process takes a shorter period of time than the prior conventional solvent soaking methods, it subjects the entire substrate to extremely low temperature and thus significant stress. The McCune process may also require a drying step to remove the condensation after the substrate is removed from the nitrogen bath. McCune also does not provide a method to dispose of the metal after removal which may result in the nitrogen bath becoming saturated with metal particles and thus cause the potential for metal redisposition on the substrate.
U.S. Pat. No. 4,631,250 to Hayashi teaches a process for removing covering films from the surface of a substrate by blasting the film with a spray of CO2 particles. In order to achieve the required force and abrasive properties to remove the film layers, the CO2 is cooled and accelerated by mixing same with N2 gas at the spray source. The CO2 particles may be further mixed with fine ice particles to increase the abrasive properties of the spray. The method disclosed in Hayashi may be suitable for removing a baked photoresist mask from a substrate surface, however, the CO2 particle bombardment disclosed in Hayashi would be far too damaging to the metal conductor portions since would tend to undesirably and indiscriminately remove the metal conductor portions given its force and abrasive properties.
In light of the prior art described above, there is a need for an improved process to selectively remove the metal layer accumulated during lift off patterning process while not damaging the conductor portions.
It is an object of the invention to provide an improved process to selectively remove the metal layer accumulated on the photoresist layer during the wafer patterning process while not damaging the deposited the metal conductor portions.
In accordance with an aspect of the invention, there is provided a process for selectively removing a metal layer accumulated on a photoresist layer during substrate patterning process comprising the following steps:generating a stream of high pressure CO2 snow; providing a substrate including a photoresist layer, a metal film layer on top of said photoresist layer and metal conductor portions; said metal film layer and metal conductor portions being of like composition and being discontinuous from each other; said metal conductor portions being in contact with said substrate surface; applying said stream to said metal film layer thereby rapidly cooling said metal layer and causing portions of said metal layer to debond, lift and peel from said photoresist layer while leaving said metal conductor portions intact.
The application of said stream to said metal layer causes thermal shock to said photoresist layer producing a plurality of cracks in the surface of said metal layer. The cracks typically including at least one loose metal edge.
Preferably further steps are provided as follows: directing said stream at said cracks and preferably said metal edge such that said stream causes further portions of said metal layer to debond, lift and peel away from said photoresist layer; and continuing application of said spray to said substrate until the entire metal layer is removed from said photoresist layer.
The further step of further continuing application of said spray to said photoresist layer, thereby removing a surface layer of said photoresist layer and leaving a remaining resist layer may also be provided. Preferably, continued treatment of the surface with CO2 snow particles erodes the photoresist and thus removes any heavily cross-linked layers created during the metalization process.
In accordance with yet a further aspect of the invention, preferably the temperature of the substrate is raised and maintained at significantly higher than environmental temperature. By keeping the substrate at this elevated temperature, the CO2 spray will cause removal of the metal film layer upon the photoresist layer and encourage the photoresist layer to be removed while keeping the metal conductor portions attached directly to the substrate in tact given that the metal attached directly to the substrate has good thermal contact and thus maintains its temperature, whereas the metal and photoresist layers do not and thus are caused to cool and differentially shrink, and/or crack and peel and thus be removed.
The portions of metal layer and/or photoresist layer removed may be collected in a filter.
Preferably after the metal layer and the surface layer of the photoresist are removed, a photoresist strip such as a simple chemical strip, such as acetone, NMP, or a dry ashing process completely removes all the remaining photoresist, leaving the substrate with undamaged conductor portions.
An advantage of the invention is that the process causes rapid removal of the metal film layer which accumulates on the photoresist layer during the forming of patterned metal conductor layers on substrates without damaging the metal conductor portions. This is achieved without the inclusion of additional materials such as an additional metal layer for causing microcracks in the metal layer, nor without exposing the entire substrate to extreme stress such as that caused by immersion in an N2 bath or subjecting the entire wafer to low temperatures. A further advantage is that the process eliminates the metal stringers and veils usually associated with metal lift-off of thick metal films or extremely narrow and closely spaced metal lines. The process also removes the heavily cross-linked skin layer, textures, and induces cracks in the underlying photoresist, thus promoting faster solvent absorption and rapid removal of the photoresist. When incorporating the dry ashing process above as the photoresist strip, the invention provides the significant advantage of being a completely dry process.
Further features of the invention will be described or will become apparent in the course of the following detailed description.